The invention relates to the manufacture of aluminium alloy products and semi-finished products, such as rolling ingots, extrusion billets or sheets, from recycled raw materials, such as machining scrap. The invention particularly relates to the manufacture of products and semi-finished products made of 7000 series alloys according to the Aluminium Association and the European standard EN 573-3 (or any other specification), i.e. Alxe2x80x94Znxe2x80x94Mg and Alxe2x80x94Znxe2x80x94Mgxe2x80x94Cu alloys, particularly those intended for the aeronautical industry.
In several industries, finished metal product manufacturing processes including such operations as machining, processing, cutting, etc. of intermediate products generate high quantities of machining scrap. In the text below, the term xe2x80x9cscrapxe2x80x9d refers to the excess material removed from intermediate products during manufacturing processes including such operations as turning, slicing, planing, milling, surfacing, trimming, drilling, tapping, extrusion, sawing, boring or machining finishing operations or similar operations. The term xe2x80x9cscrapxe2x80x9d refers to both small pieces (or xe2x80x9cpiecemeal scrapxe2x80x9d), such as chips, and larger pieces, such as cutting scrap or cutting outlines of thin or tick sheets, profiles, ingots, billets, etc.
For practical and cost reasons, particularly in the case of xe2x80x9cpiecemealxe2x80x9d scrap (i.e. small pieces of scrap, such as chips), the scrap is generally put away in bulk, with no distinction made between alloys. As a result, although, in principle, all aluminium-based scrap is recyclable, it is often downgraded, and therefore depreciated, and recycled as xe2x80x9crun-of-the-millxe2x80x9d aluminium. However, in certain industries, such as the aeronautical industry, the intermediate metal products have a high intrinsic value mainly due to the actual composition of their specified alloy, such that the depreciation of scrap increases the production costs of finished products. Moreover, the composition of these alloys, which is often special, may not be compatible with their recycling in wrought alloys containing lower amounts of alloying elements, particularly in wrought alloys belonging to other families. This gives rise to technical and economical limitations during the smelting of these alloys when scrap of aeronautical alloys of the 7000 series is to be used.
The aeronautical industry uses a large number of aluminium alloys, essentially found in the 1000, 2000 and 7000 series of the Aluminium Association and the European standard EN 573-3. For example, in the 7000 series alone, this industry particularly uses 7010, 7040, 7050, 7075, 7150, 7175 and 7475 alloys. Under these conditions, it is difficult to envisage, for cost, organisation and stock management reasons, storing scrap made of these alloys separately. Therefore, machining scrap stocks are generally very heterogeneous and of a variable composition. In addition, even if it were planned to sort the different alloys contained in the scrap, it would be difficult to prevent any accidental pollution of scrap stocks made of a given alloy by scrap from other alloys of the same series or different series.
Moreover, the composition of alloys differs significantly, even within a given series of alloys, which generally makes it impossible to recycle machining scrap combined to manufacture products made of a specific loaded alloy. Thus, the industrial practice in processing plants consists of manufacturing such products only using alloys produced from relatively pure bases or low alloy products whose composition is corrected by adding specified alloy elements and/or using listed large pieces of scrap made of specific alloy having nominal composition from the plant""s own production, such as plate stripping layers or rolling rejects. This industrial practice ensures great control of the specific loaded alloys, which particularly makes it possible to prevent random modifications of the composition of the products obtained.
This problem is of particular importance for 7000 series alloys intended for the aeronautical industry, particularly due to the large number of grades used. Firstly, the relatively complex composition of these alloys must be maintained within narrow ranges. Secondly, these alloys have very different zirconium and chromium contents, which are anti-recrystallising elements, the content of which must be observed in the final alloy.
Therefore, the applicant attempted to find a manufacturing process for intermediate products made of 7000 series alloys according to the Aluminium Association or CEN (EN 573-3) standard or any other specification, using scrap from products made of alloys in this series, which retains the advantages of the current industrial practice and is not a redhibitory burden for the manufacturing costs.
According to the invention, the manufacturing process of an intermediate product such as a rolling ingot, extrusion billet, forging block or ingot, made of a specified 7000 series alloy (according to the Aluminium Association, the European standard EN 573-3 or any other specification), with said specified alloy having a target content of at least one first anti-recrystallising element, is characterised in that it comprises:
the supply of scrap comprising scrap made of at least one second 7000 series alloy having a target content of at least one second anti-recrystallising element greater than the maximum accepted content in said specified alloy;
at least one refining step of said scrap to reduce the content of said second anti-recrystallising element to a value below the maximum accepted content in said specified alloy;
the production of a batch of liquid metal having nominal composition, in part or in whole, from the purified metal obtained through the refining step;
the formation of said intermediate product by casting said metal having nominal composition.
The word xe2x80x9cscrapxe2x80x9d encompasses aluminium or aluminium alloys materials resulting from the collection and/or recovery of metals arising at various stages of fabrication or of intermediate or finished products after use, to be used for the production of aluminium alloys. The term xe2x80x9cnew scrapxe2x80x9d designates scrap arising from the production and fabrication of aluminium products while the scrap arising from products after use is termed xe2x80x9cold scrapxe2x80x9d.
Within the scope of the present invention said scrap is typically constituted of new scrap arising from the fabrication of finished products by machining, working, cutting, etc. of said intermediate product. Said new scrap is most often essentially constituted of machining scrap (that is constituted of at least 90%, or even 95%, machining scrap), such as machining chips or turnings. Said new scrap may also comprise scrapped products, particularly discarded products such as damaged intermediate products (e.g. products damaged during production, handling or transportation). Said scrap may also include recovered and/or collected products arising from various stages of fabrication of the intermediate product and/or from its subsequent use, such as new or old scrap.
The invention is preferably applied when the first anti-recrystallising element is chromium (Cr) and the second anti-recrystallising element is zirconium (Zr). The invention is also applicable to the opposite case where the first anti-recrystallising element is zirconium and the second anti-recrystallising element is chromium.
The applicant observed that, contrary to expectations, it was possible to directly recycle batches essentially comprising scrap made of 7000 series alloys, but of a random average composition, within the scope of a relatively simple process, without producing deviations in the composition of the processed products, by means of the introduction of at least one refining step during the production of the metal having nominal composition. It was also surprised to note that it was possible to partially, and selectively, refine an alloy as loaded as a 7000 series alloy.
The process according to the invention does not require the classification of scrap by alloy; it is sufficient for the scrap to be all made of 7000 series alloys, even if the compositions of the alloys of this series differ significantly. This makes it possible to recycle scrap from different 7000 series alloys together.
In addition, the process makes it possible to recycle scrap from 7000 series alloys easily even if the average composition of the supplies varies considerably, which is normally the case since it depends on the respective quantities of different alloys used in the plant in intermediate products made and the fraction of intermediate products rejected in the form of scrap.